For the most part, telephone central office switch equipment employed throughout the United States, such as that diagrammatically illustrated at 10 in FIG. 1, is provided with a no test trunk, or NTT 11. NTT 11 is ported to an associated test bus 12 that is interfaced with the subscriber telephone lines served by the central office via a switch arrangement 14, as well as a set of access ports respectively comprising Tip (T), Ring (R), Sleeve (S) and Ground (G) leads, that allow for installation of a line-conditioning and/or test device 13. The central office access device is directly accessible by a craftsperson from a remote location, allowing the craftsperson to perform prescribed conditioning or test functions on any subscriber line 15.
Where the accessing device is a DATU, the craftsperson is able to test the line by invoking prescribed key combinations (sequences) from a craftsperson's telephone test set, without the need for participation by central office personnel. As a non-limiting example, the DATU may be of the type described in the U.S. patent to A. Chan et al, U.S. Pat No. 4,841,560 (hereinafter referred to as the '560 patent), entitled: "Direct Access Test Unit for Central Office," issued Jun. 20, 1989, and the disclosure of which is herein incorporated. Advantageously, the DATU described in the '560 patent is operative to respond to the craftsperson selectively keying in a specified audio tone combination from a telephone test handset, coupled to T/R pair 17 and to synthesize voice instructions to the craftsperson.
In some central office installations, the central office switch may not include an NTT, so that the above-described T, R, S and G leads are not available to directly connect to a piece of conditioning or test equipment, such as a DATU. Examples of such `non-standard` central office switches include those that have installed in a variety of networks outside the United States, such as the Ericsson ARF 101/102 switch and the Standard Electric PC-1000 switch, currently employed in Brazilian telephone exchanges.
The Ericsson ARF 101/102 central office switch 20 is diagrammatically shown in FIG. 2 as being ported via A/B leads to respective line circuit equipments 31, and contains an access interface 33 having five signalling leads: A, B, C, D and G (ground). The Standard Electric PC-1000 central office switch is diagrammatically shown in FIG. 3 as being ported via A/B leads to respective line circuit equipments 31, and contains an access interface 35 having eight signalling leads: A, B, S, S1, S2, SL, BL and G (ground).